After the war, old firearms, like old soldiers, began to fade away.
Formost among the former were muzzle-loading muskets. The 1865
Allin "Trapdoor" Conversion single-shot represented a new standard
for U.S. military armament: breech-loading metallic cartridge
rifles. Such arms fired ready-made metal cartridges that were
fitted into a chamber at the breech, which was then closed by the
hinged "trapdoor" bolt. The arm was now ready to fire, using a
metal pin to strike the cartridge primer. These arms introduced an
entirely new emphasis in firepower. Even though some were only
single-shot, breechloading arms were much faster to reload than
muzzle-loaders. Fewer men could fire more rounds in less
time."

- Dr. William L. Roberts, THE AMERICAN LIBERTY COLLECTION;
#81

By the close of the Civil War, muzzle-loading arms were hopelessly
outdated. Although the Army had adopted the both the Morse
breech-loading conversion system and its center-fire cartridge
prior to the Civil War, the outbreak of the War Between the States
stopped development and production. With the return of peace, many
inventors attempted to sell various patents to the government for
the conversion of muskets to breech-loaders. Erskine Allin,
Springfield Armory's Master Armorer, had begun developing a
breech-loading conversion system for the U.S. Model 1861
Rifle-Musket as early as 1864. His modification involved cutting
away the back of the barrel and installing a cam-locked
breechblock.

The first of these, termed the Model 1865, or First Allin, were
issued early in 1866. Problems soon developed, and resulted in a
complete redesign of both the rifle action and its ammunition. The
Model 1865's complicated and weak extraction system was replaced by
a system of springs, which proved to be far more rugged and
reliable than Allin's original design. The new .50-70 center-fire
cartridge, developed by Steven Vincent Benet of Frankford Arsenal,
represented a vast improvement in metallic cartridge ammunition for
U.S. military use and became the Army's first generally issued
center-fire round. To facilitate use of the new .50 caliber
ammunition, existing .58 caliber musket barrels were bored out and
sleeved with a brazed-in steel or wrought iron liner and re-rifled.
The new breech-loading musket was heavy, weighing over 10 pounds,
but it was very accurate and much more reliable than Allin's
earlier design. The issue of these Model 1866 "Second Allin"
Springfields caused a minor scandal in Congress.

In 1866, the Laidley Board had been convened to select a new
breech-loading rifle for U.S. service. Over a dozen arms had been
submitted, including the Remington rolling block and Sharps falling
block designs. As it was still under development at the time when
the Laidley Board convened, the Springfield Allin Conversion was
not represented. The Board eventually recommended the Peabody, a
forerunner of the British Martini-Henry, for adoption by the U.S.
Army.

When Springfield Armory began issuing a breech-loading arm that had
not been presented to the Board, well-connected firearms inventors
and their corporate sponsors stormed Washington. A second Board was
convened, and after a long investigation, the Springfield-Allin
Conversion was formally adopted for Army use. This decision may
well have been influenced by the fact that the government would not
be required to pay patent royalties on Allin's work.

The Model 1866 Allin rifles first saw action in the fall of 1867
during the "Hayfield Fight" near Fort C. F. Smith in southern
Montana, and the "Wagon Box Fight" near Fort Phil Kearny in
northern Wyoming. Both of these posts guarded the Bozeman Trail,
and these battles pitted the Sioux nations under Chief Red Cloud
against small forage parties made up of both soldiers and
civilians. The Sioux, expecting to face an enemy armed with
obsolete muzzle-loading muskets, were planning to make short work
of these bands, just as they had in the Fetterman Massacre of the
previous December. It was not to be.

In two separate day-long battles, the mixed parties of soldiers and
civilians held their attackers at bay, even though they were
outnumbered by as much as 30 to 1. Casualties among the soldiers
were surprisingly light. Even though the Model 1866
Springfield-Allin was a major improvement, it was excessively
heavy, and it continued to suffer more than its share of extraction
problems.

It was soon replaced in front line service by the Springfield Model
1868 "Trapdoor" rifle, but continued to be widely issued to scouts
and contract buffalo hunters well into the 1870s. Many of these
hunters, including William "Buffalo Bill" Cody, made their
reputations not with the Sharps "Big Fifty," but with the
Springfield-Allin rifle. At the start of the Franco-Prussian War in
1870, thousands of Second Allins were sold to France. Perhaps more
of these rifles saw service in Europe than ever fired a shot in the
American West.